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Tiny Lasers, Big Advances

December 15, 2008

While an undergraduate in the early 1980s at Creighton University in Omaha, Neb., Nan Jokerst thought lasers were so cool she should build one herself. Using plans from a Scientific American article, she did just that in the basement of the physics building. "It worked, amazingly enough," she says with a laugh, "though I nearly electrocuted myself, which wouldn't have been good for an electrical engineer."

This, her first foray into the world of laser optics also worked at a deeper level, because it helped inspire the research path she remains on to day. After getting a Ph.D. in Electrical Engineering from the University of Southern California, and spending14 years at Georgia Tech University, where she became the Joseph Pettit Chair in Optoelectronics, she arrived at Duke in 2003. In addition to a her position as the J. A. Jones Professor in the Pratt School of Engineering, Jokerst is also executive director of Duke's new Shared Materials Implementation Facility (SMIF), a far cry from that Creighton basement, given SMIF's status as one of the world's most sophisticated cleanroom and characterization facilities.

Jokerst says even in her early academic days she was interested in the idea of working with miniaturized systems, though she could hardly have imagined then the scale of today's advanced lasers. Groups of them could fit on the tip of a piece of spaghetti, and work in this realm of optics miniaturization is where Jokerst has long since emerged as a leader.

Though involved in other research, such as teaming with David Smith and Steve Cummer to create advanced metamaterials, Jokerst's main focus is on miniaturizing optics and lasers and integrating them with related technologies to create a range of innovative chip-scale sensing systems. "What I'm interested in doing is taking big systems that are useful and making them very tiny, portable, and low cost," she says. Potential applications range from early diagnosis of life-threatening diseases, including one that took the life of her uncle, to measuring pollutants and toxins in the water and air.

"For me personally, as I've gotten older, doing research that has some significance to humans and the environment has become much more important to me," says Jokerst, "That's why I do what I do. This kind of work has become almost a renewal for me in my interests."

Promising Union: Melding Lasers and Silicon Chips

Lasers are, of course, ubiquitous now, with uses ranging from medical procedures to transmitting data via the growing global network of fiber optic cables. But while miniaturization has long been a hallmark of advancing technologies, integrating miniaturized lasers with other devices has proven especially challenging, slowly the development of new applications.

Laser integration is more than an academic exercise, because it offers the promise of making certain technologies more widely available through associated shrinking of production costs by leveraging silicon chip manufacturing technology. In some cases it also enables new, more sensitive detection methods. Such benefits can, for instance, make it possible to apply advanced photonics in regions of the globe where it might not have otherwise been feasible, or to widely distribute environmental sensors to better monitor and ultimately curb pollution.

Jokerst's work is about bringing photonics down to a level comparable to integrated circuits, and coupling photonic with miniaturized technologies already more widely used. "We had really big computers in the 50s, then we went to integrated circuits that got smaller and smaller and cheaper," she says, "We're doing the same things with photonics."

Over the past 20 years, she and her colleagues have developed the methods needed to thin and integrate lasers - ten could fit on the typed period at the end of this sentence. More recently, the team became the first in the world to successfully glue these devices with waveguides and optical detectors directly onto silicon, an achievement that opens a huge range of application options.

Though other groups are exploring possibilities for use of miniaturized, integrated lasers for transmitting data, such as in PCs or servers, Jokerst remains focused on the incredible potential of the devices in sensing applications. Because the work is being done using the same basic tools employed for building silicon chips, manufacturing costs are low, a critical requirement for broader use.

Advances that Resonate: The Power of Light

While bonding lasers onto silicon was a major achievement, by themselves these lasers just make an interesting, very small light show. Obviously the key to benefiting from the lasers is effective integration with other components.

One of the first steps toward that end is channeling the light, which is accomplished using wave guides, in this case comparably thin polymer ribbons. Initial application of the flattened, or planar, lasers has focused on coupling them via waveguides to plate-shaped structures known as resonators. The resonators, which can be a third the width of a human hair on down to one thirtieth of that width, can trap some of the light coming from a laser. This causes photons of certain wavelengths to be captured in such a way that they are trapped, or resonate, within the resonator chamber. The rest of the light passes by the resonator to a detector that measures this output. The trick to using the resonators is that slight variations at their surface cause predictable changes that shift the wavelengths of light captured, leading to measurable changes at the detector. The Jokerst team is now exploiting these changes for a variety of sensing applications.

Fluid Movements: Combining Photonics with Lab-on-a-Chip Technology

Effective application of a laser-resonator device is dependant on the ability to get samples to the resonators, which is no simple task at such minute scales. So, one major area of collaborative research for Jokerst involves integrating her developments with so-called "lab-on-a-chip" techniques through collaborative work with Richard Fair, also in the Pratt School of Engineering. "Lab-on-a-chip" is a broad term for technologies that control the movement of fluids on a computer chip to perform biological and chemical analyses that once required bulky equipment.

Lab-on-a-chip devices can electrically manipulate small droplets of samples such as blood or seawater through a series of channels, adding other chemicals or splitting a sample into even smaller droplets as needed along the way.

Applications: Keeping an Eye on Air and Water Quality

As an initial target and proof of concept for her miniaturized photonics work, Jokerst first constructed systems to detect two specific chemicals -- o-xylene and toluene -- that are important potential pollutants in certain industrial activities. To do that, her team applied special polymer membranes that reliably control the flow of target chemicals from the sample to the surface of resonators. At the resonator, these chemicals shift the light wavelength that resonates, creating a telltale signal. With o-xylene for instance, they were able to detect levels of just one part per million, though compliance with Environmental Protection Agency standards would only require detection at the four parts per million level.

Similar schemes can be used to detect a range of other chemical pollutants. For this side of their work, the Jokerst lab has been collaborating with O’Brien Gere/Source Sentinel, one of the world's largest water quality companies, with efforts focused on monitoring industrial effluent outflows. "Most companies really want to know what is coming out of their pipes," says Jokerst, "because they don't want to be responsible for something that's an accidental emission." To simplify the monitoring process, Jokerst and her team are taking yet a further step to integrate systems with wireless technology, allowing remote monitoring.

The same general detection scheme can also be applied to a huge array of chemicals that can cause problems in waterways or the ocean. Altered versions of the scheme should also allow detection of biological components such as harmful bacteria or algal toxins responsible for major scourges such as the red tides that afflict Florida and New England.

When dealing with effluent, and to an even greater degree with seawater, one major barrier to real-world use is that detectors quickly become clogged with algae or particles. One possibility will be to design systems with flushing capabilities. But Jokerst says an even simpler solution may be to essentially succumb to the clogging, an option that becomes attractive considering the economy of the devices her team is creating. Because they are targeting a cost of just a few dollars for each sensor, it would not be prohibitively expensive to build, for instance, an array of 100 sensors in a system programmed to shift to a new sensor every three days or so, or at whatever timescale is needed to beat the clogging. Remarkably, with the integration and miniaturization that Jokerst is doing, these sensors would all fit on a dime.

Such sensor arrays would mean a system that could be deployed on an ocean buoy for about a year without servicing, an option that would be extremely attractive to oceanographers faced with the difficulty of accessing remote automated monitoring systems or with the challenges of limited funding for expeditions to these remote areas. Avoiding clogging problems would be equally attractive for many other applications "There's huge potential here," says Jokerst.

The Jokerst lab is also exploring modifications to the water sensing work that would allow similar detection of air pollutants.

Applications: Spotting Disease In Time to Prevent It

After proving their selective membrane systems, Jokerst and her team have set out to conquer the medical realm, working in collaboration with Dr. Debra Schwinn in Genomics at the University of Washington and Dr. Ralph Corey in Infectious Diseases at Duke University’s Global Health Program. An initial focus for these collaborations, and one of the most exciting potential applications for this line of research, is creating economical field tests for diseases such as malaria.

One of the greatest challenges in fighting the spread of some diseases, especially in developing countries, is simply identifying those people who have the disease early enough to save them, and, in the case of infectious diseases, early enough to minimize their exposure to other people. Testing for malaria can be especially challenging because while the parasite that causes the disease can be identified under a microscope, determining if it is a deadly strain, or a drug resistant one, requires more complicated and expensive analyses.

Developing a simple, cheap test for deadly, and drug-resistant malaria could therefore save countless lives, and the Jokerst group has that as a target for work funded by the Keck Foundation. The goal is to create a reliable test that costs just $2. The basic plan, which could be applied for the detection of a wide range of diseases and could be even more sensitive than the best existing lab tests, is to use a unique variation on the same general idea that has driven the water quality testing project.

Instead of using selective membranes to drive targeted chemicals to resonator surfaces for detection, Jokerst is working with Schwinn to attach to the resonators DNA strands that match telltale genetic sequences in the organism or substance of interest. In the case of malaria, that would be a snippet of the DNA sequence for the deadly parasite.

Prior to reaching the resonator, a sample drop of blood can be heated to denature, or unzip, the DNA present. If a sample contains the parasite, its DNA would bind with the strand on the resonator, altering the flow of light to the detector. A more generalized parasite sequence could be used for detection, or snippets from a particular strain such as one showing drug resistance.

This DNA technique might also be employed in the fight against emerging influenzas, such as bird flu. Rapid development of sensors that detect emerging infectious diseases is critical, which poses an even greater problem given that the disease is highly contagious. Multiple diseases might also be detected using the same chip system.

Early detection could allow identification of flu carriers long before they showed signs of full-blown sickness. Not only would this make it possible for them to receive treatment in time to prevent suffering and even death, it would also allow officials to sequester carriers to prevent further spread of the disease. "If you can spot the disease before people become truly infectious then you're way ahead of the game in stopping a pandemic flu," says Jokerst.

Still another potential application would be in detecting sepsis, a deadly condition caused by a bacterial infection of the blood that often isn't detected until a patient shows symptoms of the condition, which may be too late for it to be controlled. Such work would be especially close to Jokerst's heart, because her own uncle died just 48 hours after being diagnosed with sepsis. Early diagnosis of the many different bacterial strains that can cause the condition again offers the potential to save numerous lives. "This kind of sensor," says Jokerst, "could be used daily to catch sepsis before it’s too late – it’s critical when you are vulnerable."